176 VEGETABLE PHYSIOLOGY 



in. a soil, can be made soluble in the hygroscopic water 

 permeating that soil, and can dialyse through the semi- 

 permeable membrane of the root-hair, absorption of a 

 certain quantity of it will take place. How much is ul- 

 timately absorbed is a question of the power of the plant 

 to decompose or utilise it after absorption. Many sub- 

 stances which are useless or even deleterious to the 

 plant which takes them up are absorbed continuously 

 until a very large percentage of them is present, because 

 other constituents of the plant decompose them, or because 

 their power of dialysis is such that they are easily removed 

 from the absorbing cells. The possibility of the dialysis 

 by which they are originally taken up is perhaps a ques- 

 tion of relationship between the size of their molecules 

 and that of the meshes of the protoplasmic membranes 

 which bound the cytoplasm on its two faces, abutting on 

 the cell-wall and the vacuolar cavity respectively. This 

 possibility of penetrating these membranes, and the power 

 of subsequently removing the substances from the absorbing 

 cells, are the special features of the so-called selective power 

 of the plant, and it is evident that this power is particu- 

 larly associated with the disposition of the materials after 

 absorption, more than with the absorption itself. 



We may now turn to the consideration of these varied 

 constituents of the ash, and examine them in detail. The 

 first group, we have seen, is composed of sulphur and 

 phosphorus. Its importance lies in the fact that these 

 elements enter into very close relationship with protoplasm, 

 the former at any rate being in all probability a constituent 

 of its molecule. 



Sulphur is only taken up by the higher plants in the 

 form of sulphates of some of the metals of the other groups 

 or of ammonia. Fungi can also utilise salts of sulphurous 

 and hydrosulphurous acids when they are presented in 

 dilute solutions. 



Phosphorus is associated with the nucleus rather than 

 with the cell-protoplasm. It is contained in the substance 



